Late payment - what charges/penalties can I make?

Hi,

I am registered as self-employed and work in media as a freelancer. I am owed money by one company for 2 seperate invoices - one from June and one from September. Both are for several hundred pounds. I specfied on the invoice that they were to be paid within 30 days.
I was wondering what can I charge as a penalty to ensure that I get paid soon? Is it £40 + 8% per day?
Can it be backdated to any invoice that was paid late(even if it has now been paid), as almost every invoice I have sent the company has been paid significantly late?
Does a penalty charge have to be specified on an invoice, or would I leagally be entitled to charge it regardless?

Thanks